Localization is the ability for a mobile robot to know its position at all times. When the initial position is unknown, the localization process has to manage several possible positions that could correspond to the real one. The main drawback of this technique is the cost of computational complexity that could be high. In this paper, we present a new way to determine the set of initial possible positions that is fast (less than 3s) and enables to start the localization process with a small number of possible positions. The consequence is that our localization process determines the real position in a fast and robust way. Experimental results show the benefits of the method.